The issue is especially severe if you have some kind of explicit voting which allows people to register an opinion without having to create actual content to go with it.
See, for example, http://us.imdb.com/Ratings?0172495 for one such voting system. See http://slashdot.org/moderation.shtml (the section entitled "Who") for some discussion of one approach.
See also SockPuppet for a more informal version. See VotingIsEvil for other problems with votes.
This problem can be reduced if you can know the "age" or "maturity" of the login. You can't know who is the real owner of the login, but you can know how the login is used. Then you can restrict voting to the logins matching some conditions. It doesn't eliminate the problem, because people can still operate multiple accounts such that each account matches the relevant conditions. A related problem was "carpetbagging" of building societies in the UK: many people deposited exactly £5,000, in order to exploit the voting system.
Therefore, don't give all votes equal weight. Instead, weight votes according to some metric that is roughly proportional to the time, effort, or resources that the voter has invested in the community. Thus there would be no more reason to create multiple identities. For example:
A RealWorld parallel is votes undertaken by stockholders, which are weighted according to the number of shares owned by each stockholder.
Would people see such a money-based system as unfair or inappropriate? Can we always get a good metric? Would focusing power in the hands of the most obsessed members of a community distort its broader needs?
In the RealWorld, it's trivial to avoid stuffing the ballot box because people have to physically show up to vote with ID that is fairly difficult to falsify. Here, the instance of a person is tied to their physical body in a one-to-one relationship.
So, basically the problem is that you want to tie something online to something physical. You theoretically can't do that using just online tools. Even if you forced people to log in, people could still have multiple login accounts. You can't tie log ins to IPs either because the people<->IP relationship is many to many.
I suppose you could accomplish this "easily" in a sci-fi future where every citizen is given a government verified public/private key pair. "Easily" is still not perfect because you can use man in the middle attacks or packet spoofing or whatever to subvert the authenticity of the key authority.
You can use other information that is tied to a person that you can authenticate, like their credit card, but who is seriously going to give their credit information so they can vote on a movie?
You might be able to use driver's licenses, but internationally authenticating driver's licenses is probably a nightmare. However, similar man-in-the-middle or spoofing attacks are possible here. You'll never really get away from that anyway; you can only make it practically hard to crack, say by using strong encryption. -- SunirShah
In most places in the United States, you do not need any ID to vote. In general, US citizens are not required to have ID at all. A driver's license, for instance, is only necessary if you want to drive, and only when you're actually driving. It's part of the idea that citizens have a right to not be watched by their government. -- KatherineDerbyshire